In work holding chucks of the type having replaceable top jaws mounted on and detachably fastened to master jaws, it is conventional to fasten the top jaws by hold-down screws that may extend through holes in the top jaws and into T-nuts disposed in radial T-slots provided in the master jaws, or alternatively, by hold-down screws that typically spread split dovetail members in dovetail slots. In the case of the dovetail arrangement, the fact that the hold-down screws extend through holes in the top jaws reduces to that extent at least the area of frictional engagement between the dovetail members and the slots. Also, in both of these constructions, the T-slots or dovetail slots as the case may be extend radially of the master jaws so that, when the hold-down screws are loosened, the top jaws can be adjusted radially in accordance with the size of workpiece to be held thereby.
Precise radial positioning of the top jaws conventionally is achieved by set screws at one end or the other of the jaws. In these chucks, the top jaws are held primarily, if not entirely, by friction between the T-nuts or dovetail members and the slots. Consequently, any loosening of the hold-down screws in use due to vibration or otherwise results in a lessening of clamping pressure that may cause the work to fly out of the chuck and result in possible injury to persons and damage to equipment in the vicinity of the chuck.
Top jaws mounted and secured in the manner described above are replaceable so that the chuck can be adapted for different kinds and forms of workpieces. However, the jaw change operation is tedious and time consuming since it requires that all of the hold down screws be removed and then replaced. Since the screws and other parts of the jaw assemblies are completely disassembled they are liable to be misplaced or even lost during the changeover operation. Further, while the adjustable stops permit limited radial adjustment of the top jaws for different diameter workpieces, the adjustment is time consuming since the chuck has to be tested for runout after each adjustment.
Thus, there is a need in the art for a top jaw mounting that holds the top jaws securely in use but that also permits them to be easily and quickly removed and replaced.